This invention relates to image processing apparatus and, more particularly, to such apparatus which employs a convolution filter for processing pixels which constitute a video image.
Image processing techniques have long been used to modify video signal information so as to produce special effects, enhancements and visual modifications of an image produced from a video signal. Advances in integrated circuit technology, such as LSI and VLSI technology, have made it possible to provide circuits that achieve two-dimensional image processing relatively inexpensively and efficiently. Such image processing is referred to generally as two-dimensional processing because it produces effects, enhancements and modifications in both the horizontal and vertical directions of the image.
Examples of two-dimensional image processing apparatus include a low pass filter, a high pass filter and a convolution filter. Convolution filtering of image data is achieved by defining a desired area of the image, such as an area represented by n.times.n pixels which are centered about a pixel under consideration, by weighting each of the n.times.n pixels and then by summing the weighted pixels to produce a processed pixel which is then used as the pixel under consideration. Typically, the n.times.n array is not less than 2.times.2 and usually may be a 3.times.3 or other odd-numbered array (i.e. n is an odd number).
Two-dimensional image processing by filtering may be implemented by digital filter techniques. A digital filter is readily adaptable to perform different filtering operations to achieve different effects or enhancements of different images. For example, an adaptive filter may be readily operated, or programmed, to perform one type of filtering operation when the image information supplied thereto represents a scene containing a moving picture image and to perform a different type of filtering operation when the image information represents a scene containing a still picture image.
In many applications, the use of a low pass filter for image processing achieves less than satisfactory results. This is because a low pass filter, even if it were implemented by digital techniques, operates uniformly over the entire video image, which means that certain localized image processing cannot be effected. Similarly, the use of a high pass filter makes it difficult, if not impossible, to achieve localized processing of an image. In addition, the inherent nature of a low pass filter tends to lose or degrade critical edge information; while the inherent characteristics of a high pass filter may result in what visually appears to be a rough area caused by emphasized noise components, even though the image area itself is represented by intrinsically smooth luminance transitions.
Difficulties and drawbacks also are present when a relatively simple convolution filter is used to process image information because non-linear processing and other rather complicated processing techniques are not readily achieved. A typical adaptive filter is relatively complex and, generally, is controlled monotonically. Thus, the localized processing abilities of such an adaptive filter are not easily changed. Consequently, an adaptive filter which may be used for one type of image processing is not easily converted to carry out a different type of image processing.